To start the DC/DC boost converter, the voltage difference between the input voltage and the output voltage should be small to avoid high current spikes in the inductor of the converter or even failure of the converter to start operation. A precharge circuit is thus needed to raise the output voltage in a precharge mode to a level close to the input voltage before the actual boost mode operation of the converter can be started.
In the precharge mode, when the output voltage approaches the input voltage, the precharge current decreases due to a decrease of the drain-source voltage of the power MOSFET transistor, thereby limiting the maximum allowed load during startup and furthermore increasing the duration of the precharge mode, delaying the start of the actual boost mode operation.